Contactor Life Expectancy
What does it really mean?
Contactor Life Expectancy
What does it really mean
Table of Contents
Testing Standards…………………………………………………………… 3
Finding Electrical Life on a Life Curve………………………………………… 4
Determining Service Life…………………………………………………… 5
Summary…………………………………………………………………… 7
Revised from material published in the 1986 Sprecher + Schuh manual 2200T
“Contactor selection made easy”. The 2200T manual is no longer available in print.
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The electrical life of switchgear is defined by the total number of possible operating cycles
under operating conditions. When this number is reached, the wearing parts must be replaced
or revised. Testing for electrical life expectancy is an expensive process, requiring precise
measurements and actual components for testing. The manufacturer usually does testing and
publishes the data in technical specification guides. These numbers can vary widely from
product to product, and application to application. What does this figure really mean to the
plant or machine designer? How long is this contactor going to operate before routine repairs
and down time make it a replacement cost? To determine this, two things are needed: the
expected electrical life, and the expected service life.
On Sprecher + Schuh Series CA6 contactors, the main contacts can be replaced and are
available as replacement parts. In the case of CA6, mechanical life and electrical life can mean
two very different things, as the contacts can be replaced over and over. However, this is not
possible on the many smaller series contactors, so the electrical life applies to the entire device.
Sprecher + Schuh contactors have published electrical life data from 100,000 operations for a
definite purpose contactor to 1.3 million electrical operations on a general purpose midrange
size contactor. These numbers are the result of stringent quality testing in the factory.
Testing Standards
To insure accurate testing, the specifications of various utilization categories are used for the
testing criteria. The results of these tests are published in contactor life curves. Life load curves
are shown in Sprecher + Schuh literature for the most frequent applications, including the
following example (Table 1).
IEC Utilization
Category
UL/CSA
Ratings
Definition
AC-1
Continuous
Current
Resistance Furnaces
(Does not include
lighting loads)
Non inductive or slightly inductive loads,
Resistive Furnaces
AC-2
Essentially
an obsolete
technology
Slip-ring motors
Starting and stopping of running motors
AC-3
Inductive HP
rated
Squirrel-cage motors
Starting and stopping of running motors
(Breaking FLA)
AC-4
Inductive HP
rated
Squirrel-cage motors
Starting, plugging, and inching (Plugging
is understood as stopping or reversing
the motor rapidly by reversing the motor
primary connections while the motor is
running. Inching [or jogging] is understood
as energizing a motor once or repeatedly
for short periods to obtain small
movements of the driven mechanism.)
AC-15
Amps for
Electromagnets
general purpose
and control
relays
Electromagnets for contactors, valves,
solenoid actuators
Table 1: IEC Utilization Categories
The electrical life of a device, determined under test conditions for the various utilization
categories, is expressed in a graph as a function of the rated operational current. These values
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Contactor Life Expectancy
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provide a sound general basis for selecting contactors. In practical operations, any deviation
of the main influencing factor – the breaking current – will be on the safe side. After startup, the motor usually carries a smaller current than its rated operational current, and in a
prolonged inching phase, the break-away starting current is already slightly reduced. This
usually compensates for the influence of unfavorable circumstances which may not have been
observed.
With these graphs, the electrical life for any application (for example, inching of motors
with very high or very low starting currents), including any form of mixed operations can be
determined.
Contact Life
(Millions of Operations)
Ue = 230…400…460 VAC
CA7-97
CA7-85
CA7-60
CA7-43
CA7-37
CA7-30
CA7-23
CA7-16
10
CA7-12
CA7-9
Switching of squirrel-cage motors while starting
CA7-72
Finding Electrical
Life on a Life Curve
1
0.1
1
10
100
Rated operating current Ie AC-3 [A]
Figure 1
Figure 1 shows the Life load curve for Series CA7 contactors for utilization category AC-3 up
to 460V AC. Using this graph we can determine the electrical life of a contactor based on the
application specifications it will be used for.
Example: Given a squirrel-cage motor 3-phase 10 HP, 380 V, 16 A, AC-3 (switched-off only
when the motor is running). What is the expected contact life of a CA7-16 contactor?
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• Step 1: Locate the rated operating current of the contactor along the bottom of the graph
(x-axis).
• Step 2: Follow the bold (orange) line for the contactor up until it breaks off to the left.
Follow this imaginary horizontal line to the left. The y-axis is the contactor life in millions
of operations.
At standard operating levels, CA7-16 has an electrical life expectancy of 1.3 million
operations. The contactor electro-mechanical data is published in Sprecher + Schuh catalogs
in the technical information sections for each series. Now that we can determine what the
manufacturer’s test has shown the electrical life of the contactor to be, we can translate that
number into a real lifetime, taking into consideration how many operations we need per hour,
per week and per year.
The service life of electrical contacts depends on the number of operating cycles per hour and
the total number of operating cycles. By knowing the tested electrical life plus the number of
operations to be performed per hour/ per week/ per shift, the contactor life in approximate
real time can be determined, not just a vague number in the millions. Figure 2 illustrates how
this real time life expectancy can be determined from the total operating cycles determined by
the manufacturer and the number of operating cycles per hour the unit is expected to perform.
Figure 2 makes the following assumptions regarding shift operation
Determining
Service Life
• 1 shift operation: 50 weeks/year, 44 hours/week, 2200 hours/ year
• 2 shift operation: 50 weeks/year, 88 hours/week, 4400 hours/ year
• 3 shift operation: 365 days/year, 24 hours/day, approx. 8800 hours/ year
Figure 2
By knowing two factors of the table we can find the unknown factor. Let us find the following
unknown factors.
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Example 1: What is the number of permissible operations per hour if the planned service life
is 20 years under a single shift operation. The manufacturer’s rated electrical life is listed at 1.2
million operating cycles.
• Step one: Start at the known life
of the contactor (1.2) just above
the 1 on the Y-axis of the table.
Follow the imaginary horizontal
line to the right until it enters
the Service Life area for 20 years
under 1 Shift (shaded in orange
in Figure 3).
• Step two: Now move down the
imaginary vertical line towards
the X-axis, Number of operating
cycles per hour. The approximate
number of cycles per hour for
this selection would be 26. If this
is insufficient for the application,
proceed to Example 3 with the
desired number of operations per
Figure 3
hour.
Example 2: What is the expected service life of a contactor with a published electrical life of
1.3 million operations, operating on 1 shift at 4 cycles per hour? See Figure 4.
• Step one: Find the known life of
the contactor (1.3) on the Y-axis
of the table.
• Step two: Find the desired
operations per hour (4) on the
X-Axis.
• Step three: Follow the vertical
line from 4 up until it intercepts
the imaginary horizontal line
for 1.3 (red lines in Figure 4).
The two lines intersect above the
maximum Service Life line (in
orange). The life expectancy is
over 32 years.
Figure 4
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Example 3: What is the required life to operate a contactor at 20 cycles per hour on 2 shifts
with a service life desired for 12 years? See Figure 5.
• Step one: Locate the number
of cycles per hour on the X-axis
(20).
• Step two: Follow the vertical
line up until it interposes within
the service life area for 12 years
under 2 shifts (orange area in
Figure 5), approximately between
8 and 16 years.
• Step three: Follow the horizontal
line to the left to find published
life. The approximate number of
operations required to meet the
desired life and operations per
hour is 1 million.
Figure 5
Summary
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While estimating the life expectancy of a specific contactor under specific operating
specifications can be difficult, by using the known or desired factors involved for the product,
and applying them to the chart, the expected service life in real time can be estimated. This
turns a vague number of operations into a real expectation of how long your product should
operate under normal conditions.
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